The present invention relates to a developing roller for use in a developing apparatus such as a copying machine, a facsimile machine or a laser beam printer, and a method of producing the same. More particularly, the present invention relates to a developing roller for use in a non-magnetic one-component toner developing apparatus, and a method of producing the same.
Conventionally, a developing system which uses a magnetic two-component toner has prevailed in a developing apparatus such as a laser beam printer. However, in view of recently increasing concern about the global environmental preservation and the material saving, attentions are being paid on a developing system using a non-magnetic once-component toner, which does not require recovery of the toner, generating no waste toner, and which uses out all the toner in the toner cartridge. Thus, efforts have been made to put this system into practical use.
The developing apparatus based on the non-magnetic one-component toner developing system basically has a photosensitive drum, electrostatic image-forming means for forming an electrostatic image on the photosensitive drum, a developing roller rotating in contact with the photosensitive drum, a toner-supplying member consisting of, for example, a urethane sponge supply roller, for supplying a non-magnetic one-component toner onto the developing roller, and a toner-regulating member consisting of, for example, a blade made of a urethane rubber or a urethane resin, for uniformly regulating the thickness of the toner supplied onto the developing roller. In general, while the electrostatic image-forming means forms an electrostatic image on the surface of the photosensitive drum based on the predetermined image information, the toner-supplying member supplies the toner onto the surface of the developing roller, and the toner-regulating member regulates the thickness of the toner supplied on the developing roller to form a uniform thin layer of the toner on the developing roller. The developing roller, on which the thin toner layer or a uniform thickness is formed, sequentially allows the toner adhere to the electrostatic image formed on the photosensitive drum at the nip between it and the drum. In this way, the toner development is effected.
The developing roller, used in a developing apparatus of such a non-magnetic one-component toner developing system, allows the toner, which is positively or negatively charged by the frictional contact, to electrostatically adhere to its surface. The roller is constituted by an electrically conductive roller. Such an electrically conductive roller usually has a construction wherein an electrically conductive core (roller core) constituting the roller body has, on its cylindrical surface, an electrically conductive layer made of an electrically conductive material. Conventionally, as the electrically conductive material forming the electrically conductive layer, use is made of an electrically conductive rubber material containing a rubber material such as a silicone rubber, an acrylonitrile-butadiene rubber, a urethane rubber or a silicone-modified ethylene-propylene rubber, as a base, added to which is an electrically conductive material, such as carbon black or metallic powder, for imparting electrical conductivity thereto.
However, the silicone rubber and the silicone-modified ethylene-propylene rubber contains a low molecular weight siloxane therein, which migrates onto the surface, contaminating the surface of the photosensitive drum which contacts the developing roller. The acrylonitrile-butadiene rubber uses sulfur or a sulfur derivative as a vulcanizing agent, which contaminates the photosensitive drum surface. One the other hand, while the urethane rubber hardly contaminate the photosensitive drum, its volume resistivity value largely changes in accordance with the change or its environment (i.e., large in environmental dependency), lacking in practical utility.
In order to overcome the problems of the prior art rubber-based conductive layers, in particular, the contamination problem of the photosensitive drum surface, Japanese Patent No. 2504978 discloses a technique of applying a covering layer to a rubber-based conductive layer. The disclosed covering layer is made of a reaction product of a urethane resin with a fluorine-containing compound having a functional group, i.e., a fluorine-containing urethane resin. The covering layer can stop the migrating contaminants contained in the base rubber thereat, making it possible to prevent the contamination of the photosensitive drum surface by the migrating contaminants.
However, the developing roller provided with the fluorine-containing conductive layer gives rise to generation of so-called negative ghosts. For example, there occurs a problem that when an original having black regions in the background of characters is printed, the characters slightly copied in the black regions in the printed sheet.